Sex-linked recessive

Definition

Sex-linked diseases are passed down through families through one of the X or Y chromosomes. X and Y are sex chromosomes.

Dominant inheritance occurs when an abnormal gene from one parent causes disease even though the matching gene from the other parent is normal. The abnormal gene dominates.

Recessive inheritance is when both matching genes must be abnormal to cause disease. If only one gene in the pair is abnormal, the disease does not occur, or is mild. Someone who has one abnormal gene (but no symptoms) is called a carrier. A carrier can pass this abnormal gene to his or her children.

Alternative Names

Information

X-linked diseases usually occur in males. Males have only one X chromosome. A single recessive gene on that X chromosome will cause the disease.

The Y chromosome is the other half of the XY gene pair in the male. However, the Y chromosome doesn't contain most of the genes of the X chromosome. It therefore doesn't protect the male. This is seen in diseases such as hemophilia and Duchenne muscular dystrophy.

TYPICAL SCENARIOS

For a given birth, if the mother is a carrier (only one abnormal X chromosome) and the father is a normal carrier:

25% chance of a healthy boy

25% chance of a boy with disease

25% chance of a healthy girl

25% chance of a carrier girl without disease

If the father has the disease and the mother is a normal carrier:

100% chance of a healthy boy

100% chance of a carrier girl without disease

X-LINKED RECESSIVE DISORDERS IN FEMALES

Females can get an X-linked recessive disorder, but this is very rare. An abnormal gene on the X chromosome from each parent would be required, since a female has two X chromosomes. This could occur in the two scenarios below.

For a given birth, if the mother is a carrier and the father has the disease:

25% chance of a healthy boy

25% chance of a boy with the disease

25% chance of a carrier girl

25% chance of a girl with the disease

If the mother has the disease and the father has the disease:

100% chance of the child having the disease, whether boy or girl.

The odds of either of these two scenarios are so low that X-linked recessive diseases are sometimes referred to as “male only” diseases. However, this is not technically correct.

Female carriers can have a normal X chromosome that is abnormally inactivated. This is called "skewed X-inactivation." These females may have symptoms similar to those of males.